Instance-level resource read (GET /fhir/CodeSystem/{id}, /fhir/ValueSet/{id})

[wardle]

Hades currently supports FHIR REST search for terminology resources:

• GET /fhir/CodeSystem (+ POST /fhir/CodeSystem/_search)
• GET /fhir/ValueSet (+ POST /fhir/ValueSet/_search)

…but there is no instance-level read (GET /fhir/{type}/{id}). The route set in src/com/eldrix/hades/impl/http.clj has no :id-bearing route.

Scope

• GET /fhir/CodeSystem/{id}
• GET /fhir/ValueSet/{id}
• (consider GET /fhir/ConceptMap/{id})

Return the full resource as FHIR JSON, or an OperationOutcome 404 when unknown.

Tracked here after removing the README Roadmap section.